1. Technical Field
The present invention relates to a wiring structure between steps and a wiring method thereof. The invention especially relates to a wiring structure in which a step portion between steps is wired in a state that the step portion is smoothly connected by an insulating slope. Such wiring between steps includes wiring between stacked electronic components and wiring lines intersecting with each other with a step on a plane surface. The invention also relates to an improvement of a wiring method in order to achieve the wiring structure.
2. Related Art
For example, when an electronic component, such as a semiconductor device (a package), is mounted on a substrate and the like, there are many cases in which what is called an inkjet method is used as a method for forming wiring lines coupled to electrode pads of the electronic component. In the inkjet method, a conductive ink including conductive fine particles dispersed therein is discharged as fine droplets and the droplets are dried. The inkjet method can drastically increase the degree of freedom of wiring structure as compared with a conventional wire bonding method because the shape of the wiring line can be changed in units of droplet. In addition, wiring in the air, which is provided in the wire bonding method, is not required, enabling space occupied by the wiring lines to be reduced. As a result, the mounting space of the electronic component can be reduced.
In a case where the wiring lines are formed on a pad forming surface of the electronic component with the pad forming surface up, a step corresponding to a thickness of the electronic component is formed between a mounting surface of the substrate and the like and the pad forming surface of the electronic component. The inkjet method enables the wiring lines to be formed along the step. However, the wiring lines formed along the step include bent portions as many as the number of steps, possibly impairing the mechanical reliability of the wiring lines. Therefore, if the step is included between the electrode pad, to which the wiring lines are coupled, and the wiring lines, in the inkjet method, a preprocessing for smoothly connecting the steps is usually performed so as to suppress the mechanical stress applied to the wiring lines.
To be specific, as disclosed in JP-A-2006-147650, an insulating resin material having high viscosity is discharged by a dispenser method so as to connect the pad forming surface of the electronic component and the mounting surface of the substrate and the like. Then, an insulating resin slope smoothly connecting the steps is formed. Sequentially, droplets composed of a conductive ink are discharged between the steps so as to pass on the insulating resin slope. The conductive ink is dried and fired so as to form the wiring lines. This mitigates the bend of the wiring lines, highly maintaining the mechanical and electrical reliability.
If the resin slope is formed by the dispenser method, the resin material discharged and provided from the dispenser has high viscosity. Such resin material makes it hard to avoid a time delay in the start and end of the discharge. Accordingly, it is not easy to provide the resin slope with high accuracy. In addition, the resin material having high viscosity causes the resin slope to thicken (increasing the height of the resin slope). As a result, a mounting space cannot be easily reduced.
Consequently, in recent years, the method for forming the resin slope by the ink jet method has been investigated. In the case of the ink jet method, discharged droplets have low viscosity. Therefore, a surface treatment is preliminarily performed on the pad forming surface, to which the droplet to be provided, so as to control a contact angle of the provided droplet. Specifically, as shown in FIGS. 8A to 8E, for example, as the surface treatment, a liquid repellent layer 43 is formed on a mounting surface 41a of a substrate 41 and pad forming surfaces 42a of electronic components 42 so as to evenly give high liquid affinity to these surfaces (FIG. 8A). The liquid repellent layer 43 is irradiated with ultraviolet rays L for controlling a contact angle of the droplet with respect to the liquid repellent layer 43 (FIG. 8B) so as to control the contact angle to be a predetermined value. After the surface treatment, a liquid layer 50L is formed by droplets in an amount to form the resin slope (FIGS. 8C and 8D). On a resin slope 50D, which is formed by drying the liquid layer 50L, wiring lines 48 are formed for coupling first electrode pads BP of the substrate 41 and second electrode pads P of the electronic component 42 (FIG. 8E). However, the surface treatment by irradiation with the ultraviolet rays L easily causes a variation in contact angle due to an influence of conditions including various elements such as surrounding environment, and also has a problem in achieving a surface treatment that realizes even contact angles. As a result, in the case of the inkjet method, the droplets provided for forming the resin slope 50D nonuniformly wet and spread on the liquid repellent layer 43. As shown in FIG. 8D, the end portions of a liquid layer 50L, which is formed by the droplets, are unevenly formed. That is, the droplets flow out from positions where the droplets supposed to be provided and move to surfaces of the electrode pads and the like, possibly causing connection failures.